1. Field of the Invention
The present invention relates to an image forming apparatus which has, e.g., a photosensitive drum for each color and thus performs color printing.
2. Related Background Art
Conventionally, a so-called 1D (one-drum) printer which contains only one photosensitive drum and a so-called 4D (four-drum) printer (or tandem printer) which contains plural photosensitive drums have been widely known.
The 1D printer performs image formation of four colors of yellow (Y), magenta (M), cyan (C) and black (K) by rotating four times a transfer drum on which a medium called an intermediate transfer body or a sheet of paper (simply called a sheet hereinafter) is applied.
On the other hand, in the 4D printer, since the respective photosensitive drums of the four colors are provided independently, an image including four colors can be formed at a time. Therefore, if a sheet transportation speed is constant, the 4D printer can generally achieve a print speed four times as much as that of the 1D printer. However, as described above, the 4D printer is equipped with the plural photosensitive drums respectively arranged at predetermined intervals. Thus, in the 4D printer, after color image signals of red (R), green (G) and blue (B) read by a scanner or the like are subjected to a color process and then converted into color image signals of Y, M, C and K, it is necessary to delay the image signal for a time required to transport the sheet for the interval (distance) between the adjacent color photosensitive drums. For this reason, a delay memory to be used to delay the image signals is required. Here, it should be noted that the delay memory has a storage capacity according to the distance between the adjacent photosensitive drums.
On one hand, if the sheet transportation speed of the 4D printer is constant, it is necessary to shorten a so-called sheet (paper) interval which is the interval between the trailing edge of a preceding sheet and the leading edge of a succeeding sheet so as to increase the print speed.
In an apparatus which is called an MFP (Multifunction Peripheral), it is required to output through a printer image data obtained from various devices such as a scanner, a PC (personal computer), a FAX (facsimile device) and the like. In such a situation, in order to improve productivity of a system as a whole, it is required to continuously output through the printer the image data obtained from the various devices and respectively having different image sizes.
In this case, data writing and reading in unit of line to and from the delay memory are controlled on the basis of the length of a main-scan valid (or effective) interval signal corresponding to the length of the image data in a main scan direction and the length of a sub-scan valid (or effective) interval signal corresponding to the length of the image data in a sub scan direction.
However, since the conventional 4D printer is equipped merely with a single set of delay memories, even if a sheet size (image size) is changed while a series of recording operations is being performed, the main-scan valid interval signal and the sub-scan valid interval signal can not be changed until the delay memories become empty. Therefore, next data can not be stored in the delay memory until it becomes empty, and it is thus necessary to prolong the sheet interval. That is, the images respectively having the different sizes can not be simultaneously formed on the respective photosensitive drums of the four colors.
FIGS. 17A and 17B are diagrams showing the relations between the photosensitive drums and the image sizes. In FIGS. 17A and 17B, numerals 1701 and 1702 respectively denote photosensitive drums of which development colors are different from each other, numeral 1703 denotes a recording sheet (paper) P1 on which an output image of first page is recorded, numerals 1704 and 1705 respectively denote recording sheets P2 on which output images of second page are recorded, symbol d denotes an interval between the adjacent photosensitive drums, and symbol i denotes an interval between the trailing edge of the recording sheet P1 of the first page and the leading edge of the recording sheet P2 of the second page.
In FIG. 17A, the size of the recording sheet P1 of the first page is the same as the size of the recording sheet P2 of the second page, whereby the image data can be output at the short sheet interval even by using the conventional 4D printer. On the other hand, in FIG. 17B, the size of the recording sheet P1 of the first page is different from the size of the recording sheet P2 of the second page. Thus, in the above conventional 4D printer, as described above, the main-scan valid interval signal and the sub-scan valid interval signal can not be changed until the delay memories become empty, whereby the image data can not be output in the state that the sheet interval is being shortened. That is, it is necessary in this case to prolong the interval i between the trailing edge of the recording sheet P1 of the first page and the leading edge of the recording sheet P2 of the second page more than the interval d between the adjacent photosensitive drums. Incidentally, since the 4D printer contains the four photosensitive drums, the interval between the photosensitive drums at the both ends of the printer is three times the interval d between the adjacent photosensitive drums, whereby it is necessary to enlarge or prolong the sheet interval i more than three times the interval d.